The invention relates to a CMOS circuit which is capable of responding to negative going input signals without the need of a negative power supply voltage.
Generally speaking, when a negative input signal is to be processed by a CMOS circuit, a negative supply voltage having a value somewhat more negative than the most negative expected threshold voltage is applied to the p-well and the source of the input device. That technique acts to place the device at a more negative reference than the incoming negative signal such that the incoming signal is positive with respect to the devices input threshold potential, similar to normally biased CMOS circuits.
A background patent of interest is U.S. Pat. No. 4,259,686 entitled, Circuit for Producing a Polarity-Reversed Voltage with Opposite Polarity To That of A Power Supply Voltage, by Y. Suzuki, et al. In the patent there is generally discussed, in the background of the invention, the state of the art regarding back-gate biasing as applied to MOS-FET devices and its affect upon the voltage threshold of the device. The circuit of that patent recognizes that the polarity of the back-gate bias must be opposite to that of the power supply voltage and therefore the circuitry, through means of a capacitor, transistor devices and input clocks, produces a polarity-reversed voltage opposite to that of the power supply to provide the back biasing potential.
Another patent of interest is U.S. Pat. No. 4,326,136 entitled, Threshold Circuit Using Complimentary Field Effect Transistors, by C. J. P. F. LeCan, et al. The circuit of the patent changes the threshold value of the standard fabricated CMOS device by the application of a direct voltage between the gate electrodes of the first and the second field-effect transistors. The applied direct voltage has the same polarity as the supply voltage and enables a smaller input signal swing to cause the output signal to change from one extreme value to the other extreme value. An optimal effect is achieved if the direct voltage between the gate electrodes is of the same order of magnitude as the supply voltage minus the sum of the threshold voltages of the first and the second field effect transistors.